This disclosure relates generally to semiconductor-based photovoltaics, and more particularly, to single junction and multi-junction solar cells.
A solar cell, also referred to as a photovoltaic cell, is a semiconductor device capable of converting light energy into electrical energy through the photovoltaic effect. Solar cells employ one or more P-N junctions that produce corresponding one or more electric fields, which sweep photo-generated carriers toward the terminals of the solar cell device for collection, and developing electrical current. The cost of a solar array comprising numerous solar cells is generally proportional to the area employed by the solar cells themselves, that is the area subjected to light collection. High-efficiency solar cells are of great interest since their high energy density allows for a decrease in the area required to generate a given amount of power, thus improving the cost of electricity derived from solar energy. With a high efficiency solar cell, a higher power density requires less area, resulting in a superior system cost and moving toward fossil-fuel parity. Fossil-fuel parity may be defined as the point at which photovoltaic electricity is equal to or less costly than fossil fuel based electricity sources.
The use of multi-junction devices designed to absorb and collect different portions of the solar spectrum is an effective way to achieve a highly efficient solar cell. Solar cells formed of single-junction devices may have a maximum theoretical efficiency of approximately 31%, while solar cells formed of multi-junction devices may have a maximum theoretical efficiency of 87%. However, multi-junction devices, such as triple junction devices, are more complicated and costly to fabricate. For example, the particular materials utilized for such devices may be more difficult to synthesize, and the fabrication tolerances may be increased. Moreover, many multi-junction solar cell designs require expensive substrates, or rely on substrates that result in further fabrication difficulties and increased cost, or substrate re-use, which increases fabrication costs and decreases yield.
In some further optimized solar cell designs, semiconductor compositional grades may be employed to mitigate adverse conditions at interfaces of adjacent semiconductor layers, as part of the solar cell. For example, compositional grades may be employed to reduce surface recombination losses outside the depletion region, adjacent the window layer. This may be demonstrated with AlXGa(1-X) As emitter and window layers, for example, as well as outside the depletion region in thin CuInXGa(1-X)Se2 devices. This approach may be used in a compositional grading layer between a p-In0.5Ga0.5P emitter layer and P-In0.5Al0.5P window layer in the p+-n In0.5Ga0.5P solar cell. Certain compositional grades may also be used near a heterojunction located at the window layer within the depletion region. For example, a thin, graded InXGa1-XN region may be used to minimize the valence band discontinuity between an InGaN emitter layer and the GaN window layer. Generally, these exemplary compositional grades are employed to counteract the adverse impact of semiconductor layer interfaces, as part of the solar cell device, and have generally been used in very thin, specific parts of the device stack.
What is needed is a multi-junction solar cell incorporating semiconductor materials that result in bandgaps and semiconductor compositions, which provide optimal combinations of high-efficiency and manufacturability. By enhancing the ability to manufacture solar cells with relaxed design tolerances and simplified fabrication techniques while still offering a wide range of high efficiency, the dollar-per-watt cost of solar cells may be reduced. Also needed is a solar cell design that can employ certain features, such as compositional grades, to accelerate carriers in certain key parts of the solar cell device as they are generated. The compositional grades may exist in the base and/or emitter region, the back-surface field or buffer regions, or other suitable regions disclosed herein or elsewhere, alone or in combination. The ability to use compositional grades may provide additional design flexibility in the optimal combination of high-efficiency, material thickness and relaxed design tolerances. This advantage of dollar-per-watt design flexibility may lead to achieving the best overall combination of cost and performance.